There are known duplex printing apparatuses provided with a circulating transfer route including a sheet reversing path, and adapted to transfer a sheet printed on the front side in a circulating manner along the circulating transfer route, reversing from front side to rear side, permitting a printing on the rear side. There is a recent desideratum for printing apparatuses to implement a high output in duplex printing.
For printing apparatuses, the output is affected mainly from an image forming speed at a printing mechanism, as well as sheet transfer speeds in a transfer mechanism of print sheets. There is a technique disclosed in the patent document 1 (Japanese Patent Application Laid-open Publication No. 2005-280897), which controls transfer speeds of sheets on the way of circulating transfer in duplex printing in accord with the sheet size, independently from a sheet transfer speed when printing, thereby permitting an enhanced output.